The invention will be described with reference to optical waveguides. However it will be appreciated by those skilled in the art that the inventive principles are applicable to the fabrication via photolithography of other devices or objects composed of a polymer curable via a free radical process, e.g. microlens arrays or microfluidic channels. In particular, the inventive principles are important if precise shape control is required.
There are several methods known in the art for fabricating optical waveguides composed of polymeric materials on a substrate. One commonly used method involves moulding and/or embossing, e.g. U.S. Pat. No. 5,985,084. Another involves photolithography followed by an etching process (such as reactive ion etching or plasma etching), e.g. U.S. Pat. No. 5,497,445. Another commonly used method relies on spatially selective refractive index changes resulting from imagewise exposure of a photo-curable material to UV light, e.g. U.S. Pat. No. 3,689,264. Closely related to this method is a “wet etch” process, where a photo-curable material is rendered insoluble by imagewise exposure to UV light, then unexposed material is removed in a subsequent “development” step by flushing with a suitable solvent, e.g. U.S. Pat. No. 4,609,252.
The wet etch method of the prior art, illustrated in FIGS. 1(a) to 1(d), is particularly favoured for fabricating optical waveguides because it has few process steps, is rapid and readily scaleable to high volume production techniques, and requires relatively inexpensive capital equipment. Importantly, it can produce waveguides with precisely positioned and extremely smooth side walls, thereby minimising excess optical loss caused by scattering. After deposition of a UV-curable polymer film 11 on a substrate 12 optionally bearing a lower cladding layer 13, the polymer film is imagewise exposed to UV light 14 through a mask 15 to produce insoluble regions 16. The remainder of the polymer film is removed with a solvent to leave patterned features 17, such as optical waveguide cores, standing on the substrate or the lower cladding layer. Finally, an upper cladding layer 18 can be deposited on top of the patterned features if required. For optical waveguide applications, the materials used for the lower cladding, polymer core and upper cladding layers are usually selected such that they are substantially transparent at the operating wavelength(s), and the lower cladding and upper cladding materials are generally selected such that their refractive indices are less than the refractive index of the polymer core material. The lower cladding layer may be omitted if the substrate material has suitable transparency and refractive index, and the upper cladding layer may be omitted in whole or in part if required. For non-waveguiding applications, only the polymer layer and the substrate may be required. The lower cladding and upper cladding layers may be composed of any material with suitable transparency and refractive index, provided their processing conditions are compatible with the substrate and polymer core materials. Usually they are UV-curable polymers similar to the polymer core layer, deposited for example by spin coating, and cured with UV light.
Photo-curable compositions generally contain at least two components: a reactive component such as a monomer, oligomer or polymer that can be polymerised or cross-linked; and a photo-initiator that initiates the reaction when exposed to radiation (usually UV light, but other forms of sufficiently energetic radiation such as visible light, electrons or X-rays may be employed). Each reactive component molecule must contain at least one substituent capable of undergoing addition polymerisation, typically an ethylenically unsaturated (i.e. C═C) group (e.g. in the case of acrylates, methacrylates, vinyl ethers and styrene) or an epoxy group. For optical waveguide fabrication in particular, free radical initiators (suitable for acrylates, methacrylates and styrene for example) are the most commonly used photo-initiators, although cationic initiators (suitable for epoxies and vinyl ethers for example) have also been used.
Photo-curable compositions used for waveguide fabrication via an imagewise exposure/solvent development process have been developed by several groups. Compositions incorporating free radical photo-initiators include those disclosed by NTT (U.S. Pat. No. 6,632,585), Corning (U.S. Pat. Nos. 6,114,090, 6,306,563, 6,512,874 and 6,162,579), AlliedSignal (U.S. Pat. No. 5,462,700) and McGill University (U.S. Pat. No. 6,054,253). Compositions incorporating cationic photo-initiators include those disclosed by NIT (U.S. Pat. No. 6,537,723), Shipley (U.S. Pat. No. 6,731,857), IBM (U.S. Pat. No. 5,054,872) and Ericsson (U.S. Pat. No. 6,002,828). AlliedSignal have also disclosed photo-curable compositions incorporating both free radical and cationic photo-initiators (U.S. Pat. No. 6,133,472), exploiting differences in the kinetics of free radical and cationic polymerisation. Many photo-curable compositions suitable for waveguide fabrication via an imagewise exposure/refractive index change process are also known. These mostly use free radical photo-initiators, for example those disclosed by Bell Telephone Labs (U.S. Pat. Nos. 3,689,264, 3,809,732 and 3,993,485), DuPont (U.S. Pat. No. 5,402,514), ICI (U.S. Pat. No. 5,104,771) and Gemfire (U.S. Pat. No. 6,724,968), although Corning have disclosed a system with both free radical and cationic photo-initiators (U.S. Pat. No. 6,599,957).
Regardless of the type of photo-initiator used, or whether a refractive index change or solvent development is used to fix the imagewise exposure, it is generally important that the photo-induced polymerisation reaction occurs only in those regions that have been exposed. Practically, there must be an efficient termination mechanism that stops the reaction at the boundaries between exposed and unexposed regions of the photo-curable material. With isolated waveguides, incomplete reaction termination will blur or roughen the interface between exposed and unexposed regions, thereby causing excessive scattering loss of propagating light. More serious problems occur in optical devices where waveguides are closely spaced (for example in a directional coupler or an array of parallel waveguides) or where waveguides converge to a vertex (for example in a Y splitter or a star coupler). In such devices, incomplete reaction termination can cause partial or complete gap filling or vertex rounding that can compromise the operation of the device. Oxygen is well known to be a highly efficient free radical scavenger, reacting rapidly with free radicals to form less reactive peroxy radicals, thereby causing reaction termination. Although several other variables (including UV intensity and exposure time, inherent monomer reactivity and photo-initiator spectral response) are known to affect the contrast between exposed and unexposed regions, most compositions that are photo-curable via free radical polymerisation rely on the presence of dissolved oxygen as a reaction terminator to assist in providing the required contrast. Additional free radical scavengers such as nitrones (U.S. Pat. No. 6,162,579) may also be added to improve the contrast.
Substrates for optical devices are frequently composed of a rigid material such as silicon, glass, or a ceramic, chosen for factors such as mechanical stability, thermal stability and a high degree of surface smoothness (to minimise scattering loss). However there are many applications where it is preferable for the substrate to be flexible rather than rigid, e.g. for flexible displays (W. A. MacDonald, “Engineered films for display technologies”, Journal of Materials Chemistry vol. 14, pp. 4-10, 2004) and flexible optical connectors (U.S. Pat. No. 6,709,607). Flexible substrates are also compatible with reel-to-reel processing, e.g. for waveguide fabrication (U.S. Pat. Nos. 5,985,084 and 6,724,968). Flexible substrates are typically composed of a plastic or polymer material, and several types of plastics, including polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), acrylates (such as PMMA) and polyimide (PI) have been used as substrates for flat panel displays. Of these, PET is by far the most widely used because it is inexpensive and widely available in large volume (for example as Melinex® or Mylar®), with high surface quality (i.e. smoothness). However since PET has a relatively low glass transition temperature (Tg˜82° C.), other types of (usually more expensive) plastic substrates such as polycarbonate or polyimide (e.g. Kapton®) may be used if thermal stability is a major concern. It should be noted however that a plastic substrate is not necessarily flexible (e.g. it may be particularly thick and/or semi-rigid), and that plastic substrates may be desirable for other reasons such as transparency, lower weight and lower cost. It will be appreciated that a vast number of plastics are known, many of which could be used as substrates for photo-curable polymers.
Because photolithography/wet etch processing with UV curable polymers is a low temperature process, it would be expected to be readily applicable to plastic substrates, so long as the chosen plastic is resistant to the solvent used in the wet development process. Surprisingly however, when using a photo-curable material comprising a siloxane polymer and a free radical photo-initiator, it was found that changing from a silicon substrate to a plastic substrate affected the polymerisation dynamics of the photo-curable material, such that fine features could no longer be patterned. There is a need then to find a method of avoiding or compensating for this change in polymerisation dynamics.
Any discussion of the prior art herein is not to be construed as part of the common general knowledge of those skilled in the art.